Cathode active material for secondary battery and manufacturing method thereof

ABSTRACT

The present invention relates to a cathode active material for a secondary battery and a manufacturing method thereof. A cathode active material, according to one embodiment of the present invention, comprises silicon-based primary particles, and a particle size distribution of the silicon-based primary particles is D10≥50 nm and D90≤150 nm. The cathode active material suppresses or reduces tensile hoop stress generated in lithiated silicon particles during a charging of a battery to thus suppress a crack due to a volume expansion of the silicon particles and/or an irreversible reaction caused by the crack, such that the lifetime and capacity of the battery can be improved.

FIELD OF THE INVENTION

The present invention relates to secondary battery technology and moreparticularly, to a negative electrode active material for a secondarybattery and a preparation method thereof.

BACKGROUND TECHNOLOGY

A secondary battery is a battery that can be charged and dischargedusing an electrode material having excellent reversibility, and atypical commercialized example thereof is a lithium secondary battery.The lithium secondary battery is expected to be applied as amedium/large-sized power source mounted on transportation means (e.g.automobiles) or used for power storage in a power supply network (e.g.smart grid), and also as a compact power source for small IT devices(e.g. smart phones, portable computers, and electronic paper).

When lithium metal is used as a negative electrode material of a lithiumsecondary battery, there is a risk that a short-circuit or explosion ofthe battery may occur due to the formation of a dendrite. Therefore,instead of lithium metal, crystalline carbon (e.g. graphite andartificial graphite), soft carbon, hard carbon, and carbon-based activematerials to which lithium can be intercalated and deintercalated, arewidely used. However, with the expansion of the applications of thesecondary battery, there is an increasing demand for higher capacity andhigher output from a secondary battery. Accordingly, non-carbon negativeelectrode materials having a capacity of 500 mAh/g or greater (e.g.silicon (Si), tin (Sn), or aluminum (Al)), which can replacecarbon-based negative electrode materials having a theoretical capacityof 372 mAh/g and can be alloyed with lithium, are receiving attention.

Among these non-carbon negative electrode materials, silicon has thelargest theoretical capacity, which is about 4,200 mAh/g, and thus thecommercialization of silicon is very important in terms of capacity.However, during charging the volume of silicon increases by about fourtimes compared to during discharge, and thus the electrical connectionbetween active materials may be destroyed or the active material may beseparated from a current collector due to the volume change duringcharging and discharging, and the progress of an irreversible reaction(e.g. formation of a solid electrolyte interface (SEI) layer (e.g.Li₂O)) due to corrosion of the active material by the electrolyte, andsubsequent deterioration in lifetime make the commercialization ofsilicon difficult. Accordingly, for commercialization of a siliconmaterial, it is required that the capacity and lifetime of a battery bemaximized while suppressing the changes in volume during charging anddischarging.

DETAILED DESCRIPTION OF THE INVENTION Technical Problem

Accordingly, a technical object to be solved in the present invention isto provide a negative electrode active material for a secondary batteryhaving high capacity and long lifetime while having high energy density,by improving irreversible capacity and suppressing changes in volumeaccording to charging/discharging, by using silicon.

Additionally, another technical object to be solved in the presentinvention is to provide a method for preparing a negative electrodeactive material for a secondary battery having the advantages describedabove.

Additionally, a further technical object to be solved in the presentinvention is to provide a lithium battery using the negative electrodeactive material for a secondary battery having the advantages describedabove.

Technical Solution

To achieve the above objects, the negative electrode active materialaccording to an embodiment of the present invention may includesilicon-based primary particles and the particle size distribution ofthe silicon-based primary particles may be D10≥50 nm and D90≤150 nm.

The silicon-based primary particles may have sphericity in the range of0.5 to 0.9.

In another embodiment, the negative electrode active material for asecondary battery may further include a silicon oxide film formed on thesilicon-based primary particles. The oxygen content relative to thetotal weight of the silicon particle cores and the silicon oxide islimited to the range of 9 wt % to 20 wt %. The purity of thesilicon-based primary particles may be 99% or greater and the particledistribution width of the silicon-based primary particles may be 1.0 orless.

To achieve the other technical object, the method for preparing anegative electrode active material according to an embodiment of thepresent invention may include: a step of providing silicon powder; astep of providing a dispersion mixture in which the silicon powder isdispersed in an oxidizing solvent; a step of applying mechanical stressenergy (Es) to the silicon powder of the dispersion mixture to formfinely granulated silicon particles in which the particle sizedistribution is D10≥50 nm and D90≤150 nm; and a step of drying theresulting product including the finely granulated silicon particles toobtain silicon-based primary particles. The step of forming the finelygranulated silicon particles may be a step of forming a chemical oxidelayer on the finely granulated silicon particles by means of theoxidizing solvent while simultaneously applying the mechanical stressenergy.

The oxidizing solvent may include water, deionized water, an alcoholicsolvent, or a mixture of two or more thereof. The alcoholic solvent mayinclude any solvent selected from the group consisting of ethyl alcohol,methyl alcohol, glycerol, propylene glycol, isopropyl alcohol, isobutylalcohol, polyvinyl alcohol, cyclohexanol, octyl alcohol, decanol,hexadecanol, ethylene glycol, 1,2-octanediol, 1,2-dodecanediol, and1,2-hexadecanediol, or a mixture thereof.

The application of mechanical stress energy may be achieved by a millpulverization process using a composition of abrasive particles alongwith the oxidizing solvent. The application of mechanical stress energymay be carried out by grinding which performs pressing and abrasionwhile simultaneously supplying the dispersion mixture between a spinningabrasive plate and a fixed plate. The mechanical stress energy may bedefined by the equation below:

${Es} = \frac{\pi*d*V}{60*\eta}$

in which π is the ratio of circumference to diameter, d is the diameterof a rotor, V is an RPM, and η is a viscosity.

Advantageous Effects of the Invention

According to an embodiment of the present invention, a negativeelectrode active material for a secondary battery may be provided, inwhich an irreversible reaction caused by cracks and/or fractures due tothe volume expansion of the silicon-based primary particles issuppressed, by suppressing or reducing tensile hoop stress generated inthe lithiated silicon during the charging of the battery by controllingthe particle size distribution of the silicon-based primary particles tobe D10≥50 nm and D90≤150 nm, thereby enabling the lifetime and capacityof the battery to be improved.

Additionally, according to yet another embodiment of the presentinvention, a method for preparing a negative electrode active materialfor a secondary battery may be provided, wherein during the productionof the silicon-based primary particles, excessive expansion of the coreof the silicon particles during charging/discharging of the battery isprevented and subsequent micronization is prevented while stableformation of a solid electrolyte interface (SEI) is induced, by applyingmechanical stress energy in a certain range to finely granulate thesilicon powder while simultaneously forming on the core of the siliconparticles a silicon oxide layer (hereinafter, a chemical oxide layer)formed on the silicon particles by a wet method using an oxidizingsolvent, or controlling the oxygen content within the silicon particles,thus contributing to the improvement of the lifetime of active materialparticles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the comparison results of particle sizedistribution of silicon-based primary particles according to theexamples and comparative examples of the present invention.

FIG. 2 is a flowchart showing a method of preparing silicon-basedprimary particles according to an embodiment of the present invention.

FIG. 3 is a cross-sectional view of an electrode employing silicon-basedprimary particles according to an embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, preferred embodiments will be described in detail withreference to accompanying drawings.

Examples of the present invention are provided to more fully describethe present invention to those skilled in the art. The followingexamples may be modified in various ways, but the scope of the presentinvention is not limited to these examples described hereinbelow.Rather, these examples are provided so that the present disclosure willbe more faithful and complete and fully convey the spirit of the presentinvention to those skilled in the art.

Additionally, in the drawings, the thickness and size of each layer areexaggerated for convenience and clarity of explanation, and the samereference numerals refer to the same elements in the drawings. As usedherein, the term “and/or” includes any and all combinations of any ofthe listed items.

The terminology used herein is used for the purpose of describingparticular embodiments and is not intended to be limiting of theinvention. As used herein, the singular forms may include the pluralforms as well, unless the context explicitly indicates otherwise.Additionally, it is apparent that the terms “comprise” and/or“comprising,” when used in this specification, specify the presence ofstated features, integers, steps, operations, members, components,and/or groups thereof, but do not exclude the presence or addition ofone or more other features, integers, steps, operations, members,components, and/or groups thereof.

The examples according to the present invention relate to theimprovement of the capacity and lifetime of a lithium secondary battery,in which silicon particles are used as a negative electrode activematerial, by controlling the range of the particle distribution ofsilicon powder, and to a secondary battery using the same. In general,it is known that when silicon particles are applied to a secondarybattery negative electrode, the drastic decrease in lifetime andincrease in the irreversible capacity are due to the large volumeexpansion and contraction of silicon particles duringlithiation/delithiation. As such, the present inventors have discoveredthat when the particle size distribution of the silicon primaryparticles is prepared so as to be small and uniform within a certainrange, it is possible to reduce the phenomenon of the irreversiblecapacity increasing and the lifetime characteristics deteriorating dueto the volume expansion in the negative electrode active material, andthereby the present inventors have derived the present invention.

According to an embodiment of the present invention, a negativeelectrode active material may include silicon-based primary particles,and the silicon-based primary particles may have a particle size ofD10≥50 nm, D90≤150 nm, in which D100 may be less than 250 nm. In thepresent invention, D10 may be defined as a size of a particle (orparticle diameter) corresponding to a volume of 10% on a cumulativesize-distribution curve with a total volume of 100%; D90 as a size of aparticle (or particle diameter) corresponding to a volume of 90% on thecumulative size-distribution curve; and D100 as a size of a particle (orparticle diameter) corresponding to a volume of 100% on the cumulativesize-distribution curve, that is, D100 represents the size of thecoarsest particle among the distributed particles. When thesilicon-based primary particles are prepared so as to have the aboveparticle size distribution range, it is possible to reduce thephenomenon of the irreversible capacity increasing and the lifetimecharacteristics deteriorating due to the volume expansion in thenegative electrode active material.

Additionally, according to an embodiment of the present invention, themedian particle diameter of the silicon-based primary particles, D50,represents a particle size (or the cumulative average particle diameter)corresponding to a volume of 50% on the particle size cumulativedistribution curve, and D50 may be 80 nm to 100 nm. When the D50 of thesilicon-based primary particles is less than 80 nm, the relativeproportion of the conductive layer or the conductive material in theform of particles in the active material slurry becomes large and thusthe battery capacity is lowered, whereas when the D50 of the particlesexceeds 100 nm, the capacity retention is lowered due to the increase ofthe irreversible capacity.

FIG. 1 is a graph comparing the particle size distribution according tothe examples and comparative examples of the present invention. For thedefinitions of the following Examples 1 to 3 and Comparative Examples 1and 2 please refer to Tables 1 and 2 shown below.

With reference to FIG. 1 , the silicon-based primary particles having aparticle size distribution according to the examples of the presentinvention have a narrower particle distribution width than those of thecomparative examples. In general, the particle distribution width may bedefined as a value of (D90−D10)/D50. According to an embodiment of thepresent invention, when the particle distribution width of thesilicon-based primary particles is 1 or less, and preferably 0.9 orless, the increase of the irreversible capacity due to the volumeexpansion can be alleviated within a negative electrode active material,and consequently the lifetime characteristics of a secondary battery canbe improved.

It can be seen that Examples 1 to 3 have superior irreversible capacityincreases and lifetime characteristics due to the volume expansion inthe negative electrode active material, compared to those of ComparativeExamples 1 and 2. Accordingly, with respect to Examples 1 to 3, it maybe preferred that the particle size distribution of silicon-basedprimary particles be D10≥50 nm and D90≤150 nm, and that D50 be between80 nm and 100 nm.

According to an embodiment of the present invention, the silicon-basedprimary particles may be prepared such that the sphericity, which isdefined by Equation 1 below, is in the range of 0.5 to 0.9 or below, andby using the silicon-based primary particles having said sphericity, anirreversible reaction caused by cracks or fractures of the siliconparticles during charging of the battery can be suppressed or reduced.Those particles which have a sphericity of less than 0.5 on the thinedge may be micronized by a plurality of charging/discharging operationsand thereby the lifetime may deteriorate. In contrast, when thesphericity is greater than 0.9, cracks or fractures are easily caused bythe tensile stress applied to the lithiated layer. The formation of anSEI layer is promoted in the silicon-based primary particles exposed bythe cracks or fractures, resulting in deterioration of the lifetime ofthe battery.

$\begin{matrix}{{Sphericity} = \frac{2\sqrt{\pi\; A}}{p}} & \lbrack {{Equation}\mspace{14mu} 1} \rbrack\end{matrix}$

Here, A is a projected area of the two-dimensionally projectedsilicon-based primary particles, and P is a perimeter of thetwo-dimensionally projected silicon-based primary particles. Thesphericity of the silicon-based primary particles may be measured fromthe images obtained from a scanning electron microscope usingcommercially available software such as ImageJ® (e.g. Imagej136).Alternatively, the sphericity may be measured by an FPIA-3000® flowparticle image analyzer manufactured by SYSMEX (Kobe, Japan).

According to another embodiment of the present invention, thesilicon-based primary particles may include a core of the siliconparticle and a silicon oxide layer encompassing the core. The siliconoxide layer may be a natural oxide layer or may include a chemicalsilicon oxide or thermal oxide that has been artificially grown using anoxygen-containing solvent (i.e., alcohols, distilled water, or peroxidecompounds). The silicon particle core may be polysilicon or a singlecrystal, and may even have a low degree of crystallinity or may beamorphous. Additionally, the silicon-based primary particles may includenot only silicon particles having a purity of 99% or greater capable ofmaximizing capacity, but also any intermetallic compound of silicon andat least one element selected from the group consisting of tin (Sn),antimony (Sb), zinc (Zn), germanium (Ge), aluminium (Al), copper (Cu),bismuth (Bi), cadmium (Cd), magnesium (Mg), arsenic (As), gallium (Ga),lead (Pb), and iron (Fe). In another embodiment, the silicon particlecore (100) may have closed pores therein.

In particular, in the silicon-based primary particles, the oxygencontent relative to the total weight of the silicon particle cores andthe silicon oxide layer may be in the range of 9 wt % to 20 wt %, andpreferably 10 wt % to 17 wt %. Within the above range of oxygen content,both the initial charging rate and the capacity retentioncharacteristics are maintained at 80% or higher, with the result thatsilicon-based primary particles suitable for commercialization may beprovided. When the oxygen content is less than 9 wt %, the effect ofsuppressing volume expansion is insignificant, and thus the capacityretention of the silicon-based active material complex is reduced toless than 80%, and the lifetime deterioration due to the volume changecannot be improved. However, when the oxygen content exceeds 20 wt %,the capacity retention characteristics are improved, but the initialcharging rate may be reduced to less than 80% thus the energy densitymay deteriorate.

FIG. 2 is a flowchart showing a method of preparing silicon-basedprimary particles according to an embodiment of the present invention.

With reference to FIG. 2 , first, silicon powder is provided (S10). Thesilicon powder is a commercially available granulated particle having anaverage diameter in the range of a few micrometers to a few thousandmicrometers. The silicon powder may be polycrystalline or a singlecrystal, but the present invention is not limited thereto. A dispersionmixture in which the silicon powder is dispersed in a liquid oxidizingsolvent is provided (S20).

In an embodiment, the oxidizing solvent is for forming a chemical oxidelayer of silicon and may be water, deionized water, an alcoholicsolvent, or a mixture of two or more thereof. The alcoholic solvent maybe any solvent selected from the group consisting of ethyl alcohol,methyl alcohol, glycerol, propylene glycol, isopropyl alcohol, isobutylalcohol, polyvinyl alcohol, cyclohexanol, octyl alcohol, decanol,hexadecanol, ethylene glycol, 1,2-octanediol, 1,2-dodecanediol, and1,2-hexadecanediol, or a mixture thereof. Preferably, the alcoholicsolvent is ethyl alcohol.

Then, to the silicon powder of the dispersion mixture, the mechanicalstress energy (Es) defined by Equation 2 below is applied in the rangeof 9 m/s to 18 m/s and thereby finely granulated silicon particles areformed (S30). Such a fine granulation process is efficient for theproduction of silicon particles with narrow particle size distribution,and when the pulverization process is performed by applying the stressenergy in the above range, fine silicon particles having a narrowparticle size distribution can be produced. The silicon-based activematerial particles according to the examples of the present inventionare controlled so as to have a particle size distribution of D10≥50 nmand D90≤150 nm. Additionally, D100 is controlled so as to be less than250 nm and D50 is controlled so as to have a range of 80 nm to 100 nm.

$\begin{matrix}{{Es} = \frac{\pi*d*V}{60*\eta}} & \lbrack {{Equation}\mspace{14mu} 2} \rbrack\end{matrix}$

(wherein, π is the ratio of circumference to diameter, d is the diameterof a rotor, V is an RPM, and η is a viscosity.)

In an embodiment, the application of stress energy to the silicon powdermay be performed by a milling method, in which the dispersion mixtureand abrasive particles are charged into a cylindrical or conicalcontainer rotating about a central axis and rotated. The abrasiveparticles may be beads including ceramic particles, metal particles, ora mixture thereof, but the present invention is not limited thereto. Theabrasive particles may apply mechanical compression and shear stress tothe silicon powder of the dispersion mixture by having an appropriateaverage size relative to the size of the silicon powder. In anotherembodiment, the application of mechanical stress energy to the siliconpowder may be carried out by grinding which performs pressing andabrasion while simultaneously supplying the dispersion mixture between aspinning abrasive plate and a fixed plate.

In an embodiment, before the step of drying the resulting product, anaging step may be additionally performed to further relieve the stressby further oxidizing the finely granulated silicon particles, bydispersing and stirring the resulting product into any one of theabove-mentioned oxidizing solvents or a mixed solution thereof (S45).Through the aging process, the residual stress of the cores and/or achemical oxide layer of the silicon particles accumulated during thefine granulation process using the prior stress energy is relieved, anda chemical oxide layer is additionally formed to increase the strengthof the chemical oxide layer, and thereby the chemical oxide layer mayfaithfully serve as a clamping layer to enable the suppression of thevolume change of the cores of the silicon particles during charging anddischarging.

FIG. 3 is a cross-sectional view of an electrode employing silicon-basedprimary particles according to an embodiment of the present invention.

With reference to FIG. 3 , the lithium battery (10) includes a positiveelectrode (13), a negative electrode (12), and a separator (14) disposedbetween the positive electrode (13) and the negative electrode (12). Thepositive electrode (13), negative electrode (12), and a separator (14)are wound or folded and accommodated in a battery container (15). Then,an electrolyte may be injected into the battery container (15) and itmay be sealed with a sealing member (16) and thereby the lithium battery(10) may be completed. The battery container (15) may be cylindrical,square, of thin-film type, etc. The lithium battery may be a lithium ionbattery. In particular, the negative electrode (12) may include thenegative electrode active material described above.

The lithium battery is suitable for applications requiring highcapacity, high output, and high-temperature operation (e.g. electricvehicles) in addition to the applications in conventional mobile phones,portable computers, etc., and the lithium battery may also be used inhybrid vehicles, etc. in combination with a conventional internalcombustion engine, a fuel cell, a supercapacitor, etc. In addition, thelithium battery may be used for all other applications requiring highoutput, high voltage, and high-temperature operation.

MODES FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in more detail withrespect to specific embodiments, but the present invention is notlimited to these embodiments.

EXAMPLES

A commercially available polysilicon powder (2 kg) having an averagediameter of about 5 μm was dispersed in an oxidizing solvent (10 kg) of100% ethanol to prepare a dispersion mixture. The bead millpulverization process in which the stress energy was controlled underthe conditions shown in Table 1 was performed to prepare thesilicon-based primary particles of the examples and comparativeexamples, and the sphericity and particle size distribution of theprepared particles are shown in Table 2 below. The purity of theanalyzed silicon-based primary particles was greater than 99%. The abovepreparation method is exemplary and the silicon particles may beprepared by other abrasive processes or by performing the exploding wiremethod on a bulk silicon material (e.g. silicon rods and wafers).

TABLE 1 Stress Rotor Size Viscosity Milling Energy (cm) V (rpm) (P) Time(hr) (m/s) Example 1 12.2 2500 100 17.5 16.0 Example 2 14.4 2500 200 229.4 Example 3 17.8 1900 100 17 17.7 Comparative 17.8 2500 100 14 23.3Example 1 Comparative 5.5 4500 300 33 4.3 Example 2

TABLE 2 D10 D50 D90 Particle Distribution Sphericity (nm) (nm) (nm)Width (D90 − D10)/D50 Example 1 0.8 50 100 150 1.00 Example 2 0.75 55 95140 0.89 Example 3 0.76 58 88 130 0.82 Comparative 0.5 30 62 105 1.21Example 1 Comparative 0.4 88 142 257 1.19 Example 2

Referring to Tables 1 and 2, as in Examples 1 to 3, when silicon powderis subjected to wet pulverization under stress energy in the range ofabout 9 m/s to 18 m/s, it can be seen that the sphericity falls withinthe range of 0.5 to 0.9, and the particle distribution width is lessthan 1 in the numerical range where D10 is 50 or greater and D90 is 150or less. Meanwhile, in Comparative Examples 1 and 2, it can be seen thatthe particle distribution is spread over a relatively wide range and theparticle distribution width exceeds 1. Additionally, in ComparativeExample 1, it can be seen that when the applied stress energy is toostrong, the process time is reduced, but the particle size cannot becontrolled so the particle distribution width exceeds 1, whereas inComparative Example 2, it is shown that when the stress energy isreduced, the desired particle size distribution cannot be obtained evenafter a considerable milling time.

Table 3 below evaluates the initial efficiency and proportion ofcapacity retention of half cells, in which silicon-based primaryparticles having the sphericity and particle size distribution of Table2 prepared according to examples and comparative examples were slurriedto prepare negative electrodes. The capacity retention results are thoseevaluated after 50 charging/discharging cycles. The reference initialcapacity is 4,200 mAh/g, which is the theoretical capacity of silicon.

TABLE 3 Particle Distri- Weight Retention bution Initial Ratio @ 50Width Sphericity Efficiency Capacity Times Example 1 1.00 0.8 88% 2455mAh/g 95% Example 2 0.89 0.75 89% 2387 mAh/g 96% Example 3 0.82 0.76 90%2410 mAh/g 98% Comparative 1.21 0.6 83% 2040 mAh/g 74% Example 1Comparative 1.19 0.5 80% 2148 mAh/g 71% Example 2

Referring to Table 3, comparison of Examples 1, 2, and 3 and ComparativeExamples 1 and 2 shows that the initial efficiencies were all maintainedat levels above 80%, which are levels enabling commercialization, andthe capacity to weight ratio was also shown to be 2,000 mAh/g orgreater. From the aspect of capacity retention, the initial efficiencieswere reduced to less than 75% in the case of particles where theparticle distribution width exceeded 1. The initial efficiencies weremaintained at levels above 95% in the case of particles where theparticle distribution width was less than 1 and the sphericity was inthe range of 0.7 to 0.8. The improvement in lifetime characteristics wasdue to the ability to alleviate the increase of irreversible capacitydue to volume expansion in a negative electrode active material when theparticle size distribution of the silicon primary particles used as thenegative electrode active material was prepared so as to be small anduniform within a certain range.

The present invention described above is not limited to exemplaryembodiments and accompanying drawings thereof, and it will be apparentto those of ordinary skill in the art that various substitutions,modifications, and changes can be made hereto without departing from thetechnical concept of the present invention defined in the followingclaims.

<Refernce Explanation> 10: lithium battery 13: anode 12: cathode 14:separator 15: battery container 16: inclusion member

The invention claimed is:
 1. A negative electrode active material for asecondary battery, wherein the negative electrode active materialcomprises silicon-based primary particles, a particle size distributionof the silicon-based primary particles is 50 nm≤D10<80 nm, 80 nm≤D50(median particle diameter)≤100 nm, and 105 nm<D90≤150 nm, the particlesize distribution width (D90−D10)/D50 of the silicon-based primaryparticles is 1.0 or less, circularity of the silicon-based primaryparticles is in a range of 0.6 to 0.8, wherein the circularity isdefined as: ${circularity} = \frac{2\sqrt{\pi A}}{P}$ where A is aprojected area of the two-dimensionally projected silicon-based primaryparticles, and P is a perimeter of the two-dimensionally projectedsilicon-based primary particles, and the silicon-based primary particlescomprise a silicon particle core and a chemical silicon oxide layerformed by an oxidizing solvent on the core.
 2. The negative electrodeactive material for a secondary battery as claimed in claim 1, whereinin the silicon-based primary particles the oxygen content relative tothe total weight of the silicon particle core and the silicon oxidelayer is limited to the range of 9 wt % to 20 wt %.
 3. The negativeelectrode active material for a secondary battery as claimed in claim 2,wherein in the silicon-based primary particles the oxygen contentrelative to the total weight of the silicon particle core and thesilicon oxide layer is limited to the range of 10 wt % to 17 wt %. 4.The negative electrode active material for a secondary battery asclaimed in claim 1, wherein the purity of the silicon-based primaryparticles is 99% or greater.
 5. The negative electrode active materialfor a secondary battery as claimed in claim 1, wherein the particledistribution width of the silicon-based primary particles is 0.9 orless.
 6. The negative electrode active material for a secondary batteryas claimed in claim 1, wherein the D100 particle size of thesilicon-based primary particles is less than 250 nm.
 7. The negativeelectrode active material of claim 1, wherein 130 nm≤D90≤150 nm.
 8. Amethod for preparing a negative electrode active material for asecondary battery, the method comprising: a step of providing siliconpowder; a step of providing a dispersion mixture in which the siliconpowder is dispersed in an oxidizing solvent; a step of applyingmechanical stress energy to the silicon powder of the dispersion mixtureto form finely granulated silicon particles comprising a siliconparticle core and a chemical silicon oxide layer formed by the oxidizingsolvent on the core, wherein a particle size distribution of the finelygranulated silicon particles is 50 nm≤D10<80 nm, 80 nm≤D50 (medianparticle diameter)≤100 nm, and 105 nm<D90≤150 nm and the particle sizedistribution width (D90−D10)/D50 is 1.0 or less, and wherein circularityof the finely granulated silicon particles is in a range of 0.6 to 0.8,wherein the circularity is defined as:${circularity} = \frac{2\sqrt{\pi A}}{P}$ where A is a projected area ofthe two-dimensionally projected silicon particles, and P is a perimeterof the two-dimensionally projected silicon particles; and a step ofdrying the resulting product comprising the finely granulated siliconparticles to obtain silicon-based particles.
 9. The method as claimed inclaim 8, wherein the oxidizing solvent comprises water, deionized water,an alcoholic solvent, or a mixture of two or more thereof.
 10. Themethod as claimed in claim 9, wherein the alcoholic solvent comprisesany solvent selected from the group consisting of ethyl alcohol, methylalcohol, glycerol, propylene glycol, isopropyl alcohol, isobutylalcohol, polyvinyl alcohol, cyclohexanol, octyl alcohol, decanol,hexadecanol, ethylene glycol, 1,2-octanediol, 1,2-dodecanediol, and1,2-hexadecanediol, and a mixture thereof.
 11. The method as claimed inclaim 10, wherein the D100 particle size of the finely granulatedsilicon particles is less than 250 nm.
 12. The method as claimed inclaim 8, wherein the application of mechanical stress energy is achievedby a mill pulverization process using a composition of abrasiveparticles along with the oxidizing solvent.
 13. The method of claim 8,wherein before the step of drying the resulting product, an aging stepis performed by dispersing and stirring the resulting product into anoxidizing solvent or a mixed solution thereof.
 14. The method of claim8, wherein 130 nm≤D90≤150 nm.